The subject matter disclosed herein relates to air conditioning systems. More specifically, the subject disclosure relates to thermal energy storage (TES) systems for air conditioning systems, the commonest forms of which use stored chilled water or ice.
Ice storage is used in air conditioning systems, for example, chiller systems, to take advantage of the large enthalpy content of a volume of frozen water. TES systems that involve a phase change are able to utilize the enthalpy of that phase change in addition to the heat capacity of the medium. A traditional TES system can be used in conjunction with an air conditioning system, for example, a chiller system, to time shift the use of energy by charging and discharging the storage medium, (e.g., ice or water), at different times. For example, the medium can be re-charged during the nighttime by the chiller system when the chiller is not typically needed to cool a space, for example, a building or portion of a building. During the daytime, at least for a portion of the daytime, the ice storage unit is discharged to assist the chiller in providing cooling to the building
The utilization of ice as a thermal energy storage medium requires the use of a brine solution circulating from the chiller through the ice storage unit to freeze the water in the unit. The brine, typically an ammonia or ethyl glycol solution, must have a freezing point below 32 degrees Fahrenheit or zero degrees Celsius. To freeze the water and charge the unit, the chiller must therefore chill the brine solution to below zero Celsius or 32 degrees Fahrenheit. Circulating refrigerant may also be used. The overall thermal efficiency of the compressor must be considered when operating to provide this lower temperature than that typically used in conventional air conditioning. Since during daytime operation the chiller is typically operated to provide cooling air at about 55 degrees Fahrenheit in a direct exchange (DX) system, it is inefficient to use the same chiller system to both make ice at 32 degrees Fahrenheit for the ice storage unit and also provide cooling air at the desired supply air temperature during normal operation of the chiller since the two different “lifts” would require different optimization.
Further, during daytime operation of the chiller, the heat from the work done by the chiller and the heat pumped from the building is discharged into the outside, ambient air. The chiller operates at a lower coefficient of performance since the “lift” needed to go from the coil entering refrigerant or chilled-water temperature to the ambient air temperature is higher than it typically would be at night when it rejects into cooler night time air.